The craft — one of twin spacecraft sent to explore distant planets — is cruising through interstellar space.

Caption

The craft — one of twin spacecraft sent to explore distant planets — is cruising through interstellar space.

Voyager I, an unmanned craft which has traveled over 11 billion miles from the sun since its launch on Sept. 5, 1977, turned 36 this month. (Its twin spacecraft, Voyager 2, celebrated its 36th on Aug. 20.) Voyager 1 was originally sent to observe Saturn and Jupiter, and is now beyond the fringes of the solar system.NASA via AP

The tireless Voyager I spacecraft, launched in the disco era and now more than 11 billion miles from Earth, has become the first man-made object to enter interstellar space, scientists said Thursday. Interstellar space, scientists now know with certainty, is dense with particles, and the place is literally hissing. Or maybe you could say it’s whistling in the dark.

“It’s almost a pure tone. Like middle C. But slightly varying, like your piano is not quite tuned right,” said Donald Gurnett, a University of Iowa physicist who has been working on the Voyager mission most of his adult life.

Scientists have long thought that there would be a boundary out there, somewhere, where the million-mile-per-hour “solar wind” of particles would give way abruptly to cooler, denser interstellar space, permeated by charged particles from around the galaxy.

That boundary, called the heliopause, turns out to be 11.3 billion miles from the sun, according to Voyager’s instruments and Gurnett’s calculations.

Voyager 1 has crossed a new frontier, becoming the first spacecraft ever to leave the solar system, NASA said Thursday. (NASA Jet Propulsion Laboratory)

Beyond the boundary, space is — perhaps counterintuitively — much denser with particles. There are 80,000 particles per cubic meter in the region where Voyager I is now, Gurnett said.

The sun’s hot ejecta — a plasma of charged particles — forms a vast bubble, known as the heliosphere. In the outer regions of the heliosphere, the particles are relatively few and far between, with just 1,000 particles per square meter in some regions, Gurnett said. But the heliosphere has an edge. Voyager I’s epochal crossing of the boundary, into the cooler, denser plasma, took place on Aug. 25, 2012, according to the new report.

This confirms earlier findings, published in three papers in Science in June, that Voyager I on that date in August 2012 had experienced a sudden drop in solar radiation and a spike in cosmic particles coming from all around the galaxy.

But the earlier data from the spacecraft had been somewhat ambiguous. The spacecraft continued to pick up magnetic signals that suggested it was still within the sun’s magnetic field. Ed Stone, the chief scientist for Voyager, suggested that Voyager I was flying through a transitional zone.

Now, however, scientists have a new set of measurements thanks in large part to a solar flare. On March 17, 2012, the sun ejected a huge mass of particles, and when those solar particles arrived at Voyager more than a year later, on April 9, they triggered oscillations in the charged particles of matter — the plasma — surrounding the spacecraft.

From the frequency of those oscillations — essentially the sound of space itself — the scientists could interpret the density of the plasma. That density, much higher than anything registered before in the outer solar system, offered compelling evidence that Voyager I had, in fact, entered the interstellar zone.

“For the first time we’ve actually measured the density of the plasma,” Stone said. He said he’s convinced by the new data that his spacecraft has fully penetrated interstellar space.

“It’s great. This is exploration. This is wonderful,” said Stone, who has overseen the Voyager project since the early 1970s.

The two Voyager spacecraft were launched in 1977. Voyager I flew by Jupiter and Saturn, the gravity of which helped slingshot the spacecraft toward the outer reaches of the solar system. Voyager I is now traveling at 38,000 miles per hour relative to the sun.

Voyager II flew near Jupiter and Saturn and then went on to pass by Uranus and Neptune. It is not quite as far from the sun as its sister spacecraft.

Although Voyager I is now in interstellar space, it hasn’t technically left the solar system. That’s because of the Oort cloud — a region of comets in orbit around the sun.

“We’ll get to the inner edge of the Oort cloud in about 300 years,” Stone said. “Of course the spacecraft will not still be transmitting then.”

The spacecraft draws power from the radioactive decay of Plutonium 238, and Stone thinks the dwindling power supply will force engineers to start turning off instruments in 2020. Voyager I probably will go dark by 2025.

Stone said the spacecraft will pass through the far side of the Oort cloud in about 30,000 years.

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